It isn't about being offended.
I don't get the impression anyone is offended. I'm not.
The poster is provocative. It brings this shit up. So people talk about it. People also make beheading puns. I enjoy both.
Some people (Hai Dave) seem to get 'offended' by ninjas talking about something seriously and are 'troubled' when it gets in the way of an uninterrupted lowest common denominator vomment stream.